Three Planets Bask in the Habitable Zone

Trappist-1 is only 0.05% as bright as the Sun, but as the solar system is very small, three of the planets still orbit in the habitable zone.

"For decades now, we have been living on the planet of Nuwa in a small solar system with seven rocky planets. On rare occasions, violent storms blow holes in the thick cloud cover, which usually shrouds the world of Shennong. When I was 20 years old, I came close to the planet, travelling in one of our spacecraft, and I saw something incredible through a hole in the clouds. A tall structure measuring several km, perhaps a space elevator or an atmospheric power station. Ever since, I have dreamt of going to that veiled planet one day. We do not dare to yet, as we cannot see the surface. But they are there. Their civilisation is ancient and extremely sophisticated. They must have observed us many years ago, as we approached the planetary system aboard our spacecraft.”

The account is pure science fiction, written by Swiss author Laurence Suhner, but the text is sensational, as it was published in one of the world’s leading scientific journals, Nature, which usually keeps straight to the facts. But now, imagination is allowed to rule because of excitement concerning the discovery of seven Earth-like rocky planets, which are orbiting a small red dwarf star only 39 light years from Earth. And three of the planets are even located in the habitable zone, where water could flow on their surfaces, so they might include life. Hence, when the discovery was published, Nature asked the writer to leap 400 years forward in time, imagining what life would be like in a colony near one of the planets.

ASTRONOMERS HAD 500 HOURS

The dwarf star has been known since 1999, but did not become really interesting until last year, when Michaël Gillon and his colleagues from the University of Liège, Belgium, aimed the 60 cm Trappist telescope at the red dwarf. The telescope is designed to look for exoplanets, which reveal their existence, when they pass in front of the star as observed from Earth. As it passes by the star, the planet reduces the light of the star, and the telescope

registers the light reduction. The first observations showed that two Earth-like planets were orbiting close to the star, whereas yet another planet was orbiting further away. The star was named Trappist-1, and the Belgian astronomers were allowed to use the Spitzer space telescope for 500 hours of further studies.

The observations paid off. The telescope revealed that a total of seven rocky planets orbit the dwarf star.

Five of the planets are the size of Earth, whereas the two last ones are smaller, sized somewhere in between Earth and Mars.

The seven are orbiting the star so close together that the distance between the two innermost planets is only 1.6 times the distance between Earth and the Moon. The innermost planet only takes 1.5 days to complete an orbit, whereas the outermost planet orbits the star in a matter of 19 days.

In comparison, the innermost planet of the Solar System, Mercury, takes 88 days to orbit the Sun.

STAR SWALLOWED THE FIRST PLANETS

The tiny system is only slightly bigger than Jupiter and its four largest moons – Io, Europa, Ganymede, and Callisto. The proximity of the dwarf star means that three of the planets might include life. The red dwarf only shines 0.05 % as brightly as the Sun, but that is more than enough to make the three innermost planets too hot. The three next ones are located in the habitable zone, where liquid water could exist. The outermost planet is probably frozen.

The great similarity to Jupiter’s moons has convinced the Belgian astronomers that the planetary system was formed in the same way as they were. Consequently, the newborn dwarf star was surrounded by a disc of dust and gas, which was so dense that it gave birth to several generations of planets. Together with lots of dust, they spiralled into the star and were swallowed one by one. Not until the disc had become so "thin" that the gravity of the dust no longer contributed to attracting the planets to the star, the existing planets were formed. At the same time, the planets’ mutual attraction